From U.S. Pat. No. 6,858,057 “The term “electrospun fibers” is recognized by those having ordinary skill in the art and includes those fibers produced by the processes of U.S. Pat. No. 3,994,258 to Simm and U.S. Pat. No. 4,230,650 to Guignard. The processes provide methods to produce fibers from either a molten polymer or a polymer in a solution that is drawn within an electrostatic field obtaining fine fibers of 2 to 5 microns.”
Electrospun fiber mats have been used in the manufacture of contact lenses (U.S. Pat. No. 7,563,396), as porous scaffold media for cell and tissue growth (U.S. Pat. No. 6,592,623; U.S. Pat. No. 7,235,295; U.S. Pat. No. 7,531,503), as sensors and biosensors with high surface area (U.S. Pat. No. 7,264,762; U.S. Pat. No. 7,485,591), as field emitting electrodes (U.S. Pat. No. 7,438,622), as porous and coating materials for implanted medical devices (U.S. Pat. No. 6,885,956; U.S. Pat. No. 6,889,166; U.S. Pat. No. 6,889,374; U.S. Pat. No. 7,115,220; U.S. Pat. No. 7,244,272; U.S. Pat. Nos. 7,416,559 and 7,413,575), as static dissipating media (U.S. Pat. No. 7,381,664), filters (U.S. Pat. No. 6,743,273; U.S. Pat. No. 6,858,057; U.S. Pat. No. 6,924,028; U.S. Pat. No. 7,008,465; U.S. Pat. No. 7,070,640; U.S. Pat. No. 7,090,712; U.S. Pat. No. 7,090,715; U.S. Pat. No. 7,179,317; U.S. Pat. No. 7,192,434; U.S. Pat. No. 7,220,271; U.S. Pat. No. 7,290,672; U.S. Pat. No. 7,270,693; U.S. Pat. No. 7,316,723; U.S. Pat. No. 7,318,852; U.S. Pat. No. 7,318,853), biodegradable absorbents (U.S. Pat. No. 7,172,765; U.S. Pat. No. 7,309,948), separators for batteries (U.S. Pat. No. 7,279,251), as electrodes for batteries and fuel cells (U.S. Pat. No. 7,229,944), as reinforcements (U.S. Pat. No. 6,265,333; U.S. Pat. No. 7,244,116), as membranes (U.S. Pat. No. 6,800,155; U.S. Pat. No. 7,109,136), and as catalyst beds (U.S. Pat. No. 6,916,758).
There are US Patents also involve polymer templating processes. U.S. Pat. No. 7,229,944 by Shao-Horn et al. describes how interconnected electrospun polymer fibers are used as a template for carbon fibers through graphitization processes, and how catalytic particles deposited into the nanofibers subsequently grow to desirable sizes. The patent does not describe the use of electrospun fibers as templates for a structure with low yet continuous porosity.
U.S. Pat. No. 7,482,287 by Khatri et al. describes a templating process in which a polymer nanofiber is coated with a sol-gel ceramic precursor. The polymer fiber is then removed and the result is a ceramic fiber of controlled diameter. The fibers so formed have no porosity, nor is interconnection a key feature of the polymer template. The resulting process could not be used to form microconduit networks due to the lack of interconnection and the apparent compacting of the void formed by removing the polymer.
U.S. Pat. No. 7,449,165 by Dai et al. describes a templating process for chromatographic columns in which structures having controlled micro- and meso-porosity are formed by templating particles. These pores are interconnected and the structure is mechanically robust, however, the porosity is not minimized while maintaining interconnection. Thus, the mechanical properties are not optimal. For chromatography applications, the required pore volume is usually not minimized, in order to reduce the overall size of the column.
U.S. Pat. No. 7,419,772 by Watkins et al. describes a templating process involving block copolymers. However, a fully interconnected structure would require a porosity in easily in excess of 20 vol % due to the features of ordered block copolymer geometry. U.S. Pat. No. 7,190,049 discloses a similar method for producing arrays of nanocylinders. These arrays do not involve extensive interconnection between individual cylinders. U.S. Pat. No. 7,189,435 describes a similar process in combination with lithographic techniques. The porosity in such a structure would not be homogeneously distributed. The resultant inhomogeneity would typically lead to inferior mechanical properties.
U.S. Pat. No. 7,345,002 by Schaper disclosed a method for replicating polymer microstructures. All such replication and transfer methods (see, for example, U.S. Pat. No. 6,849,558) involve surface topography only, and cannot be used to product a fully three-dimensional network of embedded pores. U.S. Pat. No. 7,186,355 by Swager describes compositions involving nanoscopic pathways. These pathways are not hollow and, though they can conduct ions or electrons, cannot transport nanoparticles, nor could they transport fluids rapidly.
There are patents involving photonic crystals and/or ordered nanopore arrays. These involve templating with close packed polymer structures (needed to create the regular array) and therefore involve a much higher level of porosity. U.S. Pat. No. 6,929,724 (in addition to numerous other patents and publications, for example U.S. Pat. No. 6,649,083) disclose methods for creating porous structures using colloids as templates. Colloidal templates usually produce non-interconnected pores unless the porosity is greatly in excess of 20 vol %. The sparse, interconnected, highly branched network templates formed by electrospun fibers are not stable geometries for known colloidal materials. The same distinctions apply with respect to numerous patents involving lyotropic liquid crystalline materials used during templating processes.
U.S. Pat. No. 6,176,874 discloses the use of solid free-form fabrication techniques such as Stereolithography (SLA), selective laser sintering (SLS), ballistic particle manufacturing (BPM), fusion deposition modeling (FDM), and three dimensional printing (3DP) to form vascular templates having a the characteristics of electrospun fiber templates. These techniques are limited in the spatial resolution of structures that may be produced in a practical time period, and would generally be unsuitable for continuous production of microconduit network structures.
U.S. Pat. No. 5,522,895 describes a method of using a porous but mechanically strong template for the purpose of growing bone. The porosity is explicitly stated to be from 20% to 50% (by volume), the porosity is gradually replaced by living tissue, thereby aiding in the long-term retention of mechanical properties, and the strength and stiffness of the system are specified in terms of exceeding an absolute threshold, rather than being near optimal.
Despite the wide variety of patents cited above, there is no known patent or publicly available literature that describes any embodiment below, compatible with high-speed mass production, for fabricating a microconduit network structure, that is, a series of highly interconnected pores or channels in which the total volume fraction occupied by pores is on the order of 10% or less. The low porosity of the microconduit network structure combined with the small diameter of the pores provides for significant benefits in the mechanical properties of porous structures while permitting the rapid transport or circulation of fluids within the structure.
It is to be understood that the foregoing is exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.